As a method of forming a thin film, plasma chemical vapor deposition (CVD) has been widely used. Examples of the plasma CVD have many kinds of methods, for example capacitively-coupled CVD, inductively-coupled CVD, microwave CVD, and ECR-CVD. Among them, the capacitively-coupled CVD has been most widely used. The capacitively-coupled CVD excites plasma between a pair of parallel plate electrodes. Therefore, the capacitively-coupled CVD with a simple structure can deposit a uniform film as compared to the other methods.
The capacitively-coupled CVD can form various kinds of thin films including semiconductor films, such as silicon-based thin films, SiC, GaAs, and GaN films, dielectric films, such as SiNx and SiO2 films, high hardness films, such as a diamond film, a diamond-like carbon thin film (DLC), and a BN film, and polymer films.
For example, the silicon-based thin film is used for a thin film solar cell. The thin film solar cell has a thin silicon film with thickness of several micrometers (μm) or less and has the advantage that an amount of silicon, which is a raw material, used in it is smaller than that in a bulk crystal silicon solar cell.
The characteristics of a thin film formed by the plasma CVD method depend on the plasma state during deposition. For example, parameters for controlling the plasma state are power to be supplied, pressure, and the kind and the flow rate of raw material gas. Since the potential of the substrate is lower than that of plasma, some of the ions generated in plasma are accelerated due to the difference between the potential of plasma and the potential of the substrate and then collide with the substrate (ion bombardment). For example, Japanese Patent Application Laid-Open (JP-A) No. 2-166283 discloses a method that applies a negative DC voltage to the substrate to actively increase the energy of ion bombardment, thereby controlling the stress of a SiN thin film.
JP-A No. 62-142767 discloses a technique in which, when an amorphous silicon film is formed on a substrate, radio frequency power for generating plasma is supplied to an electrode having the substrate provided thereon and a positive DC voltage is applied to the electrode.
JP-A No. 5-291150 (particularly, paragraphs 9 to 16) discloses a technique that supplies radio frequency power for generating plasma to a cathode electrode and applies a DC or AC bias voltage to the electrode. In the technique, the substrate is not provided on the cathode electrode, but is provided on an anode electrode. In addition, JP-A No. 5-291150 discloses a technique that controls the bias voltage on the basis of the floating potential of a plasma space. The floating potential indicates the cathode potential when the bias voltage is not applied and means a self-bias voltage generated by plasma. The technique disclosed in JP-A No. 5-291150 is to use ion bombardment for improving the film quality of a thin film. The bias voltage is applied to increase the cathode potential to be equal to or more than the floating potential. Then, the plasma potential increases and the strength of ion bombardment on the substrate provided on the anode electrode increases. Therefore, practically, the technique disclosed in JP-A No. 5-291150 is for application of a bias voltage equal to or more than the floating potential to the cathode electrode.